An ever increasing number of applications for video-conferencing, digital video broadcasting, and streaming of video contents over the Internet such as for video-on-demand services relies on a transmission of video information. When video data is being transmitted or recorded, a substantial amount of data has to be sent through conventional transmission channels of limited bandwidth or has to be stored on conventional storage media of limited data capacity. In order to transmit video information through conventional channels and store it on storage media, it is inevitable to compress or reduce the volume of digital data.
For the compression of video data, a plurality of video coding standards has been developed. Such video standards are, for instance, ITU-T standards denoted with H.26× and ISO/IEC standards denoted with MPEG-x. The most up-to-date and advanced video coding standard is currently the standard denoted as H.264/MPEG-4 AVC (see Non-Patent Reference 1).
The coding approach underlying most of these standards is based on predictive coding including the following main stages (a) to (d):
(a) dividing video frames into blocks of pixels in order to subject each video frame to data compression on a block-by-block basis;
(b) identifying temporal and spatial redundancies by predicting the individual blocks from previously coded video data;
(c) removing the identified redundancies by subtracting the predicted data from the video data; and
(d) compressing the remaining data by means of Fourier transformation, quantization, and entropy coding.
Current video coding standards differ in the prediction modes that are employed for predicting each macroblock. Most video coding standards use motion estimation and motion compensation to predict video data from previously coded and decoded frames (inter-frame prediction). Alternatively, block data may also be extrapolated from neighboring blocks of the same frame (intra-frame prediction). The H.264/AVC standard defines several intra-frame prediction modes that differ, for example, with respect to the reference pixels used for an extrapolation or with respect to the direction in which the pixels are extrapolated.
FIGS. 1A and 1B illustrate intra-prediction modes according to the H.264/AVC standard. As illustrated in FIG. 1A, a current block 10 of 4×4 pixels is predicted by extrapolating a set of thirteen reference pixels 20 located on and to the left of the current block 10 to be predicted. With this prediction, a predicted block corresponding to the current block 10 is generated. In order to perform the extrapolation, one out of 8 possible extrapolation directions (intra-prediction directions) shown in FIG. 1B may be selected. In addition, a DC-mode which employs a mean value of the reference pixels 20 for predicting the current block 10 may be selected.
In intra-frame prediction, spatial prediction on subblocks of sizes of 4×4, 8×8, or 16×16 pixels is performed in order to reduce the spatial redundancies existing in natural images. Pixel values of already coded surrounding blocks (pixel values of reference pixels) are used to predict the pixel values of the current block. These reference pixels are extrapolated according to predefined prediction modes. In H.264/AVC, there are 9 prediction modes for subblocks of 4×4 and 8×8 pixels, and 4 prediction modes for macroblocks of 16×16 pixels.
For subblocks smaller than 8×8 pixels, the 9 prediction modes include 8 prediction directions (cf. FIG. 1B), suitable for predicting directional structures, and a DC prediction mode. In the DC prediction mode, all pixels of the current block are predicted with a single value, which is the mean value of the surrounding reference pixels. In the directional modes, the reference pixels are repeated along the corresponding direction. According to the vertical mode, for example, reference pixels in a row immediately above the current block are vertically repeated (extrapolated). According to the horizontal mode, on the other hand, pixels in a column immediately to the left of the current block are horizontally repeated (extrapolated). The remaining modes are diagonal prediction modes derived in similar ways.    Non-Patent Reference 1: ISO/IEC 14496-10 “MPEG-4 Part 10 Advanced Video Coding”